It is well established that demand for wireless communication is on the rise. One approach to meet this increased need has been to reduce channel separation. Often at the heart of this approach, one must produce highly accurate signals with minimum sideband noise to accommodate the narrower channel spacing. As a result, a new class of oscillators referred to as high specification oscillators has been introduced that meet the strict requirements. These oscillators are characterized by having the best phase noise performance for a given bandwidth. It is known in the art, that in an oscillator, the phase noise is mainly controlled by the loaded Q of the resonant tank and the signal power associated with the oscillator. Presently, improvements in loaded Q are limited by metal losses and/or by tuning element losses which basically cannot be improved with today's technologies. A general solution is therefore obtained through a compromise in bandwidth versus phase noise. This compromise is due to the fact that the tuning element (i.e. the varactor) is the Q limiting element in the tank circuit. By narrowing the bandwidth of the VCO one can effectively decouple the varactor from the tank circuit which improves the overall loaded Q of the resonant structure.
For a single device oscillator with a given bandwidth, there exists an optimum drive signal that the oscillator must operate in order to have its lowest phase noise. This optimum drive level basically sets the current that the VCO will have to operate and therefore sets the signal power associated with the oscillator. Operation at higher current level produces saturation effects within the oscillator which in turn degrades the phase noise performance. As a result, designers are forced to compromise on these vital parameters.
It can therefore be seen that a need exists for an oscillator that enjoys simultaneous bandwidth and phase noise improvements.